At Washington Middle School there are $900$ students. If $\frac{3}{5}$ of the students are boys, what is the ratio of girls to boys? Express your answer as a common fraction.
Explanation: If $\frac{3}{5}$ of the students are boys, then $1-\frac{3}{5}=\frac{2}{5}$ of the students are girls, and the ratio of girls to boys is $\frac{\frac{2}{5}}{\frac{3}{5}}=\boxed{\frac{2}{3}}$. (Notice how we did not need to use the number 900 in our calculations, because the ratio remains constant regardless of the size of the set.)